1. Field of the Invention
The present invention relates to lightweight well cement compositions and methods of cementing subterranean zones penetrated by well bores using the compositions.
2. Description of the Prior Art
In carrying out completion operations in oil, gas and water wells, hydraulic cement compositions are commonly utilized. For example, hydraulic cement compositions are used in primary cementing operations whereby pipe is cemented in the well bore. That is, a hydraulic cement composition is pumped into the annular space between the walls of the well bore and the exterior of a pipe disposed therein. The cement composition is permitted to set in the annular space thereby forming an annular sheath of hardened impermeable cement therein. The objective of the cement sheath is to physically support and position the pipe in the well bore and bond the pipe to the walls of the well bore whereby the undesirable migration of fluids between zones or formations penetrated by the well bore is prevented.
Recently the need for better economics, higher productivity, environment protection and more efficient well operations has brought about new well drilling and completing techniques. Examples of such new techniques include the reduction of the well bore diameter (referred to as a slim hole) and extending the reservoir penetration by drilling small lateral well bores which are completed using small diameter pipe such as coiled tubing to increase the productivity of the reservoir. The performance of primary cementing operations in the smaller annular spaces in the well bores requires improved lightweight cement compositions having low viscosities so that the cement compositions can be accurately placed. In addition, the cement compositions must have good static gel strength, low rheology, high compressive strength, low fluid loss, low permeability, good chemical resistance and a broad operating temperature range. That is, the cement compositions must be suitable for use at temperatures in the range of from about 45xc2x0 F. to 270xc2x0 F.
Thus, there are needs for improved lightweight cement compositions and methods of using the compositions for cementing pipe in well bores.
The present invention provides improved lightweight cement compositions and methods of cementing subterranean zones utilizing the compositions which meet the needs described above and overcome the deficiencies of the prior art. The lightweight cement compositions of the invention are basically comprised of a coarse particulate hydraulic cement; an ultrafine particulate hydraulic cement mixture comprised of slag cement and a Portland or equivalent cement present in an amount in the range of from about 50% to about 150% by weight of the coarse particulate hydraulic cement in the composition; fly ash present in an amount in the range of from about 50% to about 150% by weight of the coarse particulate hydraulic cement in the composition; fumed silica present in an amount in the range of from about 20% to about 60% by weight of the coarse particulate hydraulic cement in the composition; hollow glass spheres present in an amount sufficient to impart a density to the cement composition in the range of from about 9 to about 13 pounds per gallon; and water present in an amount sufficient to form a slurry. The cement compositions also preferably include a fluid loss control additive present in an amount in the range of from about 0.2% to about 8% by weight of the coarse particulate hydraulic cement in the composition.
The coarse particulate hydraulic cement has a particle size no greater than about 118 microns and a specific surface area no less than about 2800 square centimeters per gram. The slag cement and Portland or equivalent cement in the ultrafine cement mixture has a particle size no greater than about 30 microns, a mean particle size of 6 microns and a specific surface area no less than about 6000 centimeters per gram.
The methods of the present invention for cementing a subterranean zone penetrated by a well bore are comprised of the following steps. A lightweight cement composition of the invention is prepared comprised of a coarse particulate hydraulic cement, an ultrafine particulate hydraulic cement mixture of slag cement and Portland or equivalent cement, fly ash, fumed silica, hollow glass spheres, sufficient water to form a slurry and optionally, a fluid loss control additive. After preparation, the cement composition is placed in the subterranean zone to be cemented and the cement composition is allowed to set into a hard impermeable mass.
When the subterranean zone to be cemented has a temperature in the range of from about 45xc2x0 F. to about 100xc2x0 F., a cement composition set accelerator is included in the cement composition. When the subterranean zone has a temperature in the range of from about 100xc2x0 F. to about 270xc2x0 F., a set accelerator and a dispersing agent are included in the cement composition. When the subterranean zone has a temperature in the range of from about 230xc2x0 F. to about 270xc2x0 F., a cement composition set retarder and silica flour are included in the cement composition. The silica flour functions to prevent set cement strength retrogression.
It is, therefore, an object of the present invention to provide improved lightweight well cementing compositions and methods.
Other and further objects, features and advantages of the present invention will be readily apparent to those skilled in the art upon a reading of the description of preferred embodiments which follows.
The present invention provides universal non-foamed lightweight cement compositions and methods of using the compositions for cementing subterranean zones penetrated by well bores. The lightweight cement compositions can be used over a broad temperature range, i.e., from about 45xc2x0 F. to about 270xc2x0 F. at densities in the range of from about 9 to about 13 pounds per gallon. The cement compositions have excellent properties including high static gel strengths, low rheologies, low fluid loss, high compressive strength upon setting, low permeability upon setting and resistance to chemical deterioration and failure due to sulfate degradation or the like.
The lightweight cement compositions of this invention are basically comprised of a coarse particulate hydraulic cement; an ultrafine particulate hydraulic cement mixture comprised of slag cement and a Portland or equivalent cement, fly ash, fumed silica, hollow glass spheres, water present in an amount sufficient to form a slurry and optionally, a fluid loss control additive.
The coarse particulate hydraulic cement can be any of a variety of hydraulic cements having a maximum particle size of about 118 microns and a specific surface area of about 2800 square centimeters per gram. Portland cement is generally preferred, and the coarse cement can be, for example, one or more of the various Portland cements designated as API Classes A-H cements. These cements are identified and defined in the API Specification For Materials And Testing For Well Cements, API Specification 10, 5th Edition, dated Jul. 1, 1990 of the American Petroleum Institute. API Portland cements generally have a maximum particle size of about 90 microns and a specific surface of about 3900 square centimeters per gram. When an API Portland cement is utilized as the coarse hydraulic cement in accordance with this invention, it is preferably API Class G cement. Other hydraulic cements which are more coarse than API Portland cement can also be used up to the maximum particle size set forth above. When more coarse cements are used, they preferably have properties which are the same or similar as API Class G cement.
The ultrafine particulate hydraulic cement mixture comprised of slag cement and a Portland or equivalent cement has a particle size no greater than about 30 microns, a mean particle size of 6 microns and a specific surface area no less than about 6,000 centimeters per gram. Preferably the ultrafine particulate hydraulic cement mixture has a particle size no greater than about 17 microns and a specific surface area no less than about 7,000 centimeters per gram, and more preferably, a particle size no greater than about 11 microns and a specific surface area no less than about 10,000 centimeters per gram. Ultrafine particulate hydraulic cement mixtures of slag cement and Portland or equivalent cement having particle sizes and specific surface areas as described above are disclosed in U.S. Pat. No. 4,761,183 issued on Aug. 2, 1988 to Clarke which is incorporated herein by reference. The preferred ultrafine cement mixture for use in accordance with this invention is comprised of slag cement and Portland or equivalent cement wherein the slag cement is present in the mixture in an amount of at least about 50% by weight of the mixture. The slag cement is more preferably included in the ultrafine cement mixture in an amount of about 65% by weight of the mixture and most preferably in an amount of about 75% by weight of the mixture.
The ultrafine particulate hydraulic cement mixture as described above in combination with the coarse particulate hydraulic cement provides high compressive strengths to the set cement compositions of this invention. The ultrafine particulate hydraulic cement mixture is included in the lightweight cement composition of this invention in an amount in the range of from about 50% to about 150% by weight of the coarse particulate hydraulic cement in the composition.
The fly ash utilized in the lightweight cement compositions of this invention is preferably ASTM Class F fly ash. The fly ash functions as a lightweight filler in the cement compositions and in combination with the coarse particulate hydraulic cement provides low permeability to the cement compositions upon setting. The fly ash is included in the cement compositions in an amount in the range of from about 50% to about 150% by weight of the coarse particulate hydraulic cement in the composition.
The fumed silica provides thickening and thixotropic properties to the cement compositions. The fumed silica is included in the cement compositions in an amount in the range of from about 20% to about 60% by weight of the coarse particulate hydraulic cement therein.
The hollow glass spheres are included in the cement compositions of this invention to make them lightweight, i.e., to provide low densities to the compositions. Particularly suitable such hollow glass spheres are commercially available from Halliburton Energy Services, Inc. of Duncan, Okla., under the trade designation xe2x80x9cSILICALITE(trademark).xe2x80x9d The hollow glass spheres are included in the cement compositions of this invention in an amount in the range of from about 21% to about 310% by weight of the coarse particulate hydraulic cement therein to provide densities to the cement compositions in the range of from about 9 to about 13 pounds per gallon.
The water in the cement compositions of this invention can be fresh water, unsaturated salt solutions or saturated salt solutions including brine and seawater. The water is included in the cement composition in an amount sufficient to form a pumpable slurry, i.e., an amount in the range of from about 128% to about 400% by weight of the coarse particulate hydraulic cement in the compositions.
The cement compositions of this invention also preferably include a fluid loss control additive. While a variety of fluid loss control additives can be utilized, a preferred fluid loss control additive is comprised of a mixture of a graft copolymer having a backbone of lignin, lignite or salts thereof and a grafted pendant group of 2-acrylamido-2-methylpropanesulfonic acid and a copolymer or copolymer salt of N,N-dimethylacrylamide and 2-acrylamido-2-methyl propane sulfonic acid. The above described graft copolymer is disclosed in U.S. Pat. No. 4,676,317 issued to Fry et al. on Jun. 30, 1987 and the above described copolymer or copolymer salt is disclosed in U.S. Pat. No. 4,555,269 issued to Rao et al. on Nov. 26, 1985, both of which are incorporated herein by reference. The mixture of the above fluid loss control additives provides a synergistic increase in fluid loss control and brings about less settling and less free water. When used, the fluid loss control additive is included in the cement compositions of this invention in an amount in the range of from about 0.2% to about 8% by weight of the coarse particulate hydraulic cement in the compositions.
As mentioned, the lightweight cement compositions of this invention can be utilized over a broad temperature range of from about 45xc2x0 F. to about 270xc2x0 F. The density of the cement compositions can be varied by varying the amounts of the hollow glass spheres included in the compositions. That is, the lightweight compositions of this invention can have a density from about 9 to about 13 pounds per gallon. The ability to vary the density is important in cementing subterranean zones in that low density cement compositions often must be used to prevent fracturing of the subterranean zones and lost circulation from taking place. The cement compositions of this invention have low rheologies whereby when required they can be pumped at turbulent flow. Pumping the cement composition at turbulent flow aids in displacing drilling fluid from the well bore. The low rheology of the cement composition also produces low friction pressure when the composition is pumped which lowers the risk of fracturing easily fractured zones or formations. When the cement compositions of this invention are utilized in the above mentioned slim hole completions, low rheology is particularly important in preventing fracturing of weak subterranean zones or formations.
When the cement composition of this invention is used in low temperature applications, i.e., applications where the zone being cemented has a temperature in the range of from about 45xc2x0 F. to about 100xc2x0 F., a cement composition set accelerator and a cement composition dispersing agent are preferably included in the composition. As will be understood by those skilled in the art, the set accelerator shortens the time required for the cement composition to set at the low temperatures involved and the dispersing agent lowers the rheology of the cement composition.
While a variety of cement composition set accelerators can be utilized, calcium chloride is presently preferred. When used, the set accelerator is included in the cement composition in an amount in the range of from about 1% to about 12% by weight of the coarse particulate hydraulic cement therein.
A variety of dispersing agents can also be utilized in accordance with this invention. A particularly suitable such dispersing agent is the condensation product of acetone, formaldehyde and sodium sulfite. Such a dispersing agent is commercially available from Halliburton Energy Services, Inc. of Duncan, Okla., under the tradename xe2x80x9cCFR-3(trademark).xe2x80x9d The dispersing agent utilized is included in the cement composition in an amount in the range of from about 0.2% to about 8% by weight of the coarse particulate hydraulic cement in the composition.
When the temperature of the subterranean zone to be cemented is in the range of from about 100xc2x0 F. to about 230xc2x0 F., the above described cement composition set accelerator is preferably included therein.
When the subterranean zone or formation to be cemented has a temperature in the range of from about 230xc2x0 F. to about 270xc2x0 F., the above described cement composition preferably includes a cement composition set retarder and silica flour to prevent set cement strength retrogression. While various set retarders can be utilized, the set retarder used in accordance with this invention is preferably a copolymer of 2-acrylamido-2-methylpropanesulfonic acid and acrylic acid at temperatures up to 250xc2x0 F. or a copolymer of 2-acrylamdio-2-methylpropane sulfonic acid and itaconic acid at temperatures above 250xc2x0 F. Such set retarders are commercially available under the trade designations xe2x80x9cSCR-100(trademark)xe2x80x9d and xe2x80x9cSCR-500(trademark),xe2x80x9d respectively, from Halliburton Energy Services, Inc. of Duncan, Okla. The set retarder functions to delay the set of the cement composition until it has been placed in the subterranean zone to be cemented. The amount of set retarder utilized increases with increasing temperature and is generally included in the cement compositions of this invention in an amount in the range of from about 0.2% to about 8% by weight of the coarse particulate hydraulic cement in the compositions.
As indicated above, silica flour is included in the cement compositions to prevent the compressive strength of the set cement from decreasing over time due to the high temperature of the subterranean zone in which it is placed. When used, the silica flour is included in the cement compositions in an amount in the range of from about 20% to about 60% by weight of the coarse particulate hydraulic cement in the compositions.
In order to prevent the chemical degradation of the set cement composition of this invention, the coarse particulate hydraulic cement utilized can optionally be an API Class G Portland cement which does not contain tricalcium aluminate. The presence of tricalcium aluminate in the cement can cause sulfate degradation of the cement.
A particularly suitable lightweight cement composition of this invention is comprised of a coarse particulate API Class G Portland cement having a particle size no greater than about 118 microns and a specific surface area no less than about 2800 square centimeters per gram; an ultrafine particulate hydraulic cement mixture comprised of slag cement and a Portland or equivalent cement, the cement mixture having a particle size no greater than about 30 microns and a specific surface area no less than about 6,000 centimeters per gram and being present in an amount in the range of from about 50% to about 150% by weight of the coarse particulate hydraulic cement in the composition; ASTM Class F fly ash or the equivalent present in an amount in the range of from about 50% to about 150% by weight of the coarse particulate hydraulic cement in the composition; fumed silica present in an amount in the range of from about 20% to about 60% by weight of the coarse particulate hydraulic cement in the composition; hollow glass spheres present in an amount sufficient to impart a density to the cement composition in the range of from about 9 to about 13 pounds per gallon; and water present in an amount sufficient to form a slurry.
The water in the composition can be selected from the group consisting of fresh water, saturated salt solutions and unsaturated salt solutions including brine and seawater, and the water can be present in a general amount in the range of from about 128% to about 400% by weight of the coarse particulate hydraulic cement in the composition.
As indicated above, the composition preferably also includes a fluid loss control additive comprised of a mixture of the graft copolymer (Halliburton xe2x80x9cSCR-100(trademark)xe2x80x9d) and the copolymer or copolymer salt (Halliburton xe2x80x9cSCR-500(trademark)xe2x80x9d) described above in an amount in the range of from about 0.2% to about 8% by weight of the coarse particulate hydraulic cement in the composition.
Further, as also described above, depending on the temperature of the subterranean zone to be cemented, one or more additional additives are preferably included in the cement compositions of this invention. The additives include, but are not limited to, a cement composition set accelerator, a cement composition dispersing agent, a cement composition set retarder and silica flour for preventing set cement strength retrogression at elevated temperatures.
The methods of cementing a subterranean zone penetrated by a well bore in accordance with the present invention are basically comprised of the following steps. A lightweight cement composition of this invention basically comprised of a coarse particulate hydraulic cement, an ultrafine particulate hydraulic cement mixture of slag cement and a Portland or equivalent cement, fly ash, fumed silica, hollow glass spheres and sufficient water to form a slurry is prepared. Thereafter, the cement composition is placed in the subterranean zone to be cemented and the cement composition is allowed to set into a hard impermeable mass.
In order to further illustrate the lightweight cement compositions and methods of the present invention, the following examples are given.